Sources of fossil fuels useful for heating, transportation, and the production of chemicals as well as petrochemicals are becoming increasingly more scarce and costly. Industries such as those producing energy and petrochemicals are actively searching for cost-effective engineered fuel feed stock alternatives for use in generating those products and many others. Additionally, due to the ever increasing costs of fossil fuels, transportation costs for moving engineered fuel feed stocks for production of energy and petrochemicals is rapidly escalating.
These energy and petrochemical producing industries, and others, have relied on the use of fossil fuels, such as coal and oil and natural gas, for use in combustion and gasification processes for the production of energy, for heating and electricity, and the generation of synthesis gas used for the downstream production of chemicals and liquid fuels, as well as an energy source for turbines.
Combustion and gasification are thermochemical processes that are used to release the energy stored within the fuel source. Combustion takes place in a reactor in the presence of excess air, or excess oxygen. Combustion is generally used for generating steam which is used to power turbines for producing electricity. However, the brute force nature of the combustion of fuel causes significant amounts of pollutants to be generated in the gas produced. For example, combustion in an oxidizing atmosphere of, for example, fossil fuels such as coal, oil and natural gas, releases nitrogen oxides, a precursor to ground level ozone which can stimulate asthma attacks. Combustion is also the largest source of sulfur dioxide, which in turn produces sulfates that are very fine particulates. Fine particle pollution from U.S. power plants cuts short the lives of over 30,000 people each year. Hundreds of thousands of Americans suffer from asthma attacks, cardiac problems and upper and lower respiratory problems associated with fine particles from power plants.
Gasification also takes place in a reactor, although in the absence of air, or in the presence of substoichiometric amounts of oxygen. The thermochemical reactions that take place in the absence of oxygen or under substoichiometric amounts of oxygen do not result in the formation of nitrogen oxides or sulfur oxides. Therefore, gasification can eliminate much of the pollutants formed during the firing of fuel.
Gasification generates a gaseous, fuel rich product known as synthesis gas (syngas). During gasification, two processes take place that convert the fuel source into a useable fuel gas. In the first stage, pyrolysis releases the volatile components of the fuel at temperatures below 600° C. (1112° F.), a process known as devolatization. The pyrolysis also produces char that consists mainly of carbon or charcoal and ash. In the second gasification stage, the carbon remaining after pyrolysis is either reacted with steam, hydrogen, or pure oxygen. Gasification with pure oxygen or steam results in a high quality mixture of carbon monoxide and hydrogen due to no dilution of nitrogen from air.
One potential source for a large amount of feed stock for gasification is waste. Waste, such as municipal solid waste (MSW), is typically disposed of or used in combustion processes to generate heat and/or steam for use directly for heating or cooling, or in turbines for power generation. Fuels derived waste streams are often called refuse-derived fuels or RDF. The drawbacks accompanying combustion have been described above, including the production of pollutants such as nitrogen oxides, sulfur oxide, particulates and products of chlorine that damage the environment.
Hydrogen chloride (HCl) (along with other acid gaseous pollutants) is currently emitted in significant quantities by utility and industrial coal-fired furnaces, as well as by municipal, medical and hazardous waste incinerators. Coal contains only traces of chlorine, but electric utility furnaces burn large amounts of coal. In the case of waste incinerators, chlorine is contained in large amounts in some plastic wastes, such as poly(vinyl chloride) (PVC) (C2H3Cl) and poly(vinylidene chloride) (PVDC) (C2H2Cl2), as well as in some food and yard wastes. During pyrolysis of PVC and PVDC, chlorine evolves mostly as HCl, in the preliminary stages of combustion under 350° C. (Panagiotou, T., Levendis, Y. A., 1996. Combust. Sci. Technol. 112, 117, 1996). Capture of HCl is important because it is harmful, corrosive and an acid rain contributor and emissions of HCl are currently regulated (Fellows, K. T., Pilat, M. J., J. Air Waste Manag. Assoc. 40, 887, 1990). Moreover, HCl, either directly, or indirectly through the production of chlorine (Cl2 by the Deacon reaction), may contribute to subsequent formation of chlorinated unburned hydrocarbons, polychlorinated dibenzo-dioxins, and polychlorinated dibenzo-furans in the furnace effluent, (Addink, R., Bakker, W. C. M., Olie, K., Environ. Sci. Technol. 2055, 1995). Thus, capture of HCl is imperative. To avoid the production of the highly toxic dioxins and furans, which form as the effluent cools, the capture of chlorinated species must take place at high temperatures, preferably above 500° C. Formation of polychlorinated dibenzo-dioxins and furans (PCDD/PCDF) occurs in the fly ash, as the effluent stream cools down to moderate temperatures (≈300° C.) (Stieglitz, L., Zwick, G., Beck, J., Roth, W., Vogg, H., Chemosphere 18, 1219, 1989).
Sulfur dioxide emissions related to industrial operations primarily occur from combustion sources and thermal processes, such as power plants (coal or oil fired), incinerators, steam generation equipment, process heaters, chemical reactors, and other similar equipment. All these emission must follow Environmental Protection Agency (“EPA”) regulations set by the 1990 Clean Air Act Amendment. Recently, as the construction of new power generation facilities is emphasized and most of the facilities have plans to use coal, a renewed and more interest in economical methods of SO2 emissions will be needed. (Wu, C., Khang, S.-J., Keener, T. C., and Lee, S.-K., Adv. Environ. Research, 8, 655-666, 2004). It is reported that more than 250 techniques for flue gas desulfurization (FGD) have been proposed or developed on a worldwide basis (Oxley, J. H., Rosenberg, H. S., Barrett, R. E., Energy Eng. 88, 6, 1991). However, relatively few of those processes are currently in use because of low efficiency (Makansi, J., Power, 137, 23-56, 1993).
Normally fuels and waste containing significant amounts of sulfur or chlorine are not preferred for combustion and gasification reactions. Significant amounts are defined as an amount that when added to a final fuel feed stock causes the final feed stock to have more than 2% sulfur or more than 1% chlorine. For example, materials such as high sulfur containing coal, used tires, carpet, rubber, and certain plastics such as PVC, when combusted, release unacceptable amounts of harmful sulfur- and chlorine-based gases. For this reason, this material is usually avoided as a source of fuel.
Over years the literature has extensively reported that chloride induced corrosion of high temperature surfaces in boilers is one of the most costly problems in the industry. This problem can result in downtime and periodic total shutdowns of the plants, which accounts for a significant fraction of the operating and maintenance cost. It leads to replacement of super-heater pendants as often as annually in some units or the costly use of higher alloyed materials to either shield the metal surfaces or serve as replacement tube material.
The corrosion problem is more severe when biomass and waste derived fuels are used due to the fact that the ash of the biomass and waste fuels has a very different composition and different melting characteristics than the ash of coal. This difference in the ash results in corrosion and chloride salts deposits on the super heater tubes and other parts being comprised in the heat transferring devices of the plants. The corrosion from chlorine begins at steam temperatures in the super-heater of approx. 480° C. (900° F.), and increases along with the temperature up to approx. 500-600° C. (930-1100° F.). This in fact limits the super heated steam temperature in biomass to energy and waste to energy, and consequently limits the power generating efficiency plants as compared to coal fired plants.
Biomass and fossil fuels often contain multiple chemical elements in different proportions that could give rise to various environmental or technological problems during or after they are used as an energy source. Such chemical elements include other halogen gases (i.e., Cl, F, and Br), nitrogen (N), trace metals and mercury (Hg). A higher content of sulfur, chorine, or fluorine, causes serious corrosion of system equipment and creates the hazardous air pollutants as discussed above. Trace elements may also form a threat to the environment or to human health (e.g., Hg, Cd, Pb, As, Cr, Tl), they may cause additional corrosion problems (e.g., Na, K, V, Zn, Pb), and lead to fouling of the turbine blades (mainly Ca) or pollute or poison any catalysts used (mainly As) or sorbents downstream. To avoid or minimize these problems these elements, and/or products formed from these elements that may be liberated or produced during or after the conversion processes (e.g., gasification, combustion), one or more suitable technologies need to be in place to reduce their presence in fuels thereof or process products (gas, liquid or solid).
The systems that have previously been developed or implemented for gas cleaning in gasification and/or combustion processes focus on the control of these pollutants in the actual fuel itself (i.e., by limiting the use of highly polluted fuels) or by controlling the release into the atmosphere by post treatments on the flue gas stream, for example the addition of sorbents. Sorbents such as hydrated lime, calcium carbonate, sodium sesquicarbonate, sodium bicarbonate, and magnesium oxide have been injected into combustion exhaust stack gases in an effort to clean the exit gases of chlorine and sulfur containing pollutants (U.S. Pat. Nos. 6,303,083, 6,595,494, 4,657,738, and 5,817,283). However, these dry sorbents optimally work at temperatures of 800° C. to about 2000° C. and thus have only been used in the exhaust stacks or combustion units. If sorbents such as limestone are used at temperatures below 800° C. then less than 20% conversion of the pollutants occur resulting sometimes in toxic products. Therefore, these sorbents are often made in slurry form and are used in semi-dry/wet and wet scrubbers, which requires more complicated systems and operate with waste water generation, leading to higher capital and operation costs. Gasification of biomass or MSW derived fuels is usually performed at temperatures at or below 850° C. Sorbents have not been heretofore mixed in solid fuel feed stocks comprising at least one component of processed municipal solid waste.
Thus, there is a need for methods that allow the use of various fuel feed stocks in combustion or gasification applications, which otherwise can not be used due to significant amounts of pollutants produced upon combustion and gasification.
It is an object of the present invention to provide engineered fuel feed stocks comprising sorbents which allow the use of waste materials that contain significant levels of sulfur or chlorine for combustion or gasification applications.
It is a further object of the present invention to provide engineered fuel feed stocks comprising one or more sorbents that can be used to control a specific pollutant, or preferably a number of pollutants at the same time. In order to achieve multiple pollutant control, a multi-functional sorbent is ideally required; alternatively, multiple sorbents could be utilized with each sorbent being selected to treat for a particular element. Selection of sorbents is dependent on a various considerations, including, but not limited to, the following: (i) fuel characteristics, essentially what type and amount of the pollutants need to be controlled by sorbent(s); (ii) operating conditions, such as reducing or oxidizing environment, temperature, pressure, and conversion technologies (e.g., fixed bed, dense fluidized bed, circulating fluidized bed, etc.); (iii) reactivity of the sorbent and characteristics of the by-products, e.g., stability, melting point, boiling point, and toxicity; and (iv) economic effectiveness.
A further object of the invention is to provide sorbent-integrated engineered fuel feed stocks with several distinct advantages, including, but not limited too, the following, improved reaction kinetics, improved sorbent reactivity, improved pollutant removal efficiency, improved fuel conversion, improved corrosion control, reduced ash slagging, reduced operating temperature, extend power generating facility lifetime, avoid expensive retrofit costs, reduced operating and maintenance costs.
With sorbents embedded within the feed stock, an intimate contact between sorbent and pollutant can be achieved where they are generated. Compared to furnace injection in which the pollutant has migrated from within fuel particles to the bulk fuel or flue gas stream, the concentration of the pollutant is higher within the particles when the sorbent is part of the fuel. This configuration improves the reaction kinetics, thus enhancing the reaction.
Further, due to the temperature gradient across the fuel particles, sintering of sorbent inside the fuel particles is reduced, and thus sorbent reactivity is higher.
Combining the sorbent with feed stock to form an integrated fuel particle also allows the use of fine sorbent particles (e.g., <1 μm) while still achieving long residence time in the reaction chamber, which could be on the order of minutes, compared to only 1-2 seconds in case of furnace injection. Together, fine sorbent particles and long residence time would significantly increase the pollutant removal efficiency.
In cases where incompletely reacted sorbents may separate from the fuel particle and get into the flue gas stream, continuous reaction with H2S (or SO2) in gas stream will continue. As a result, the sorbent utilization will be greatly improved.
Because the sorbent is part of the fuel feed stock, there is no need to have the sorbent handling systems that are normally required for dry sorbent injection systems (storage, delivery, atomizing, etc.).
Also, the products of the sorbent/pollutant reaction mostly remain in the bottom ash, therefore the particulate, or dust, load on downstream collectors (i.e., electrostatic precipitator, baghouse, particulate matter scrubber) would be reduced, resulting savings in capital, operation and maintenance costs otherwise required for these devices.
For gasification, sorbent material can also catalytically improve the fuel conversion, and thus the gasification rate and performance can be enhanced (J. Weldon, G. B. Haldipur, D. A. Lewandowski, K. J. Smith, “Advanced coal gasification and desulfurization with calcium-based sorbent”, KRW Energy Systems Inc., Madison, Pa. 15663.)